Designers and Technicians of Microwave Radio Equipment have long employed microwave attenuators to design, test, and repair microwave radios. Typically, such microwave attenuators are incorporated into a wave guide or similar housing in order to simulate adverse transmission characteristics such as, for example, transmission path fading. Generally, microwave attenuators operate by absorbing the electromagnetic energy of a wave traveling down the wave guide. The amount of attenuation may be varied by changing the insertion depth of an absorbing element.
A substantial detriment of prior microwave attenuators comprises the non-linearity of attenuation in relation to the insertion depth of an absorbing element. The non-linearity of attenuation results from a compounding of attenuation effects due to the critical parameters of absorber surface area, mass (i.e., the bulk or volume and not necessarily the weight or specific gravity of the absorber), and capacitive coupling of the absorber to the bottom of the wave guide. In FIG. 1, a rudimentary microwave attenuator 100 comprises an absorbing element 102, which may be variably inserted into a wave guide 104. Once positioned, the absorbing element 102 is fixed by a locking screw 106. As the absorber 102 is lowered into the wave guide, the inserted surface area and mass increase thereby increasing the amount of attenuation. Additionally, the capacitive coupling between the bottom edge of the absorber 102 and the bottom of the wave guide increases, which causes non-linear changes in the amount of attenuation. Therefore, small variations of the insertion depth of the absorber can result in widely varying attenuation rates of the traveling wave.
In FIG. 2, another non-linear attenuator 200 fully incorporates absorbing strips 202 within the Wave guide 206. This is possible since there is no electric field energy at the side walls of the wave guide. To vary the attenuation, an adjustment screw 204 is rotated thereby causing the absorbing strips to arch toward the center of the wave guide. As the absorbers 202 move toward the center of the wave guide 206, more of the electromagnetic energy is incident upon the absorbers 202 and the surface area and mass are gradually able to absorb this energy. This provides non-linear operation since the penetration into the center of the wave guide 206 increases the surface area and mass of the absorbers non-linearly. The capacitive coupling between the bottom edge portion of the absorbers and the bottom of the wave guide is maintained relatively constant since the absorber is always in the wave guide.
In FIG. 3a, yet another non-linear attenuator 300 is shown. The absorber 302 is rotated into a slot 304 in the wave guide 306 by rotating a knob 308. The amount of attenuation is measured by a calibrated dial 310, after which the absorber 302 is locked into position with a locking screw 312. The attenuator 300 operates non-linearly since the arcual shaped absorber 302 increases the surface area an mass non-linearly when the penetration depth of the absorber 302 within the wave guide 306 is increased. The primary advantage is that the capacitive effects are minimized for the fully inserted absorber. However, the attenuator 300 still provides non-linearly varying attenuation due to the mass and surface area effects as the absorber 302 is lowered into the wave guide 306. Typically, the calibration dial 310 is not a linear scale, but graduated in an attempt to incorporate these parameters.
The non-linear attenuator 300 of FIG. 3a may be linearized if the absorber shape is changed to that illustrated in FIG. 3b. The shape of the absorber 302' has been varied from a conventional arch to cause a shift in its point of rotation into the wave guide. However, even with this improvement, the attenuators of FIGS. 2 and 3a are expensive and bulky. Typically, these attenuators must exceeds a length of 18 inches to provide an attenuation rate on the order of 60 db. Smaller versions of these attenuators typically obtain attenuation rates not greater than 30 db. Therefore, a need exists in the art to provide a small inexpensive linear microwave attenuator that has the capability of providing improved attenuation rates over that of the prior art.